1. Field of the Invention
The present invention relates to a light emitting display, and more particularly to, a light emitting display that decreases non-uniformity in an image caused by a voltage drop in power source lines and a method of driving the same.
2. Discussion of the Background
Various thin and lightweight flat panel displays (FPD) have been developed to replace the heavier and bulkier cathode ray tubes (CRT). Such FPDs include liquid crystal displays (LCD), field emission displays (FED), plasma display panels (PDP), and light emitting displays.
Light emitting displays display images using an organic light emitting diode (OLED), which emits light by re-combination of electrons and holes. The light emitting display may have a higher response speed than a display device that requires a light source, such as the LCD.
FIG. 1 is a circuit diagram showing a pixel of a common light emitting display.
Referring to FIG. 1, each pixel 11 of the common light emitting display is arranged corresponding to a crossing of a scan line Sn and a data line Dm. Applying a scan signal to the scan line Sn selects a pixel 111 to generate light corresponding to the data signal from the data line Dm.
Therefore, the pixel 11 includes a first power source ELVDD, a second power source ELVSS, an OLED, and a pixel circuit 40.
The anode of the OLED is connected to the pixel circuit 40, and the cathode of the OLED is connected to the second power source ELVSS.
In addition to an organic light emitting layer (EML), the OLED may include an electron transport layer (ETL) and a hole transport layer (HTL) formed between the anode and the cathode. The OLED may further include an electron injection layer (EIL) and a hole injection layer (HIL). When a voltage is applied between the anode and the cathode of the OLED, electrons generated by the cathode move to the EML through the EIL and the ETL, and holes generated by the anode move to the EML through the HIL and the HTL. Therefore, the electrons and holes supplied by the ETL and the HTL recombine in the EML to generate light.
The pixel circuit 40 includes first and second transistors M1 and M2 and a capacitor C. Here, the first and second transistors M1 and M2 are p-type metal-oxide semiconductor field effect transistors (MOSFET). The second power source ELVSS may have a lower voltage level than the first power source ELVDD such as, for example, a ground voltage level.
The gate electrode of the first transistor M1 is connected to the scan line Sn, its source electrode is connected to the data line Dm, and its drain electrode is connected to a first node N1. The first transistor M1 supplies the data signal from the data line Dm to the first node N1 in response to the scan signal from the scan line Sn.
The capacitor C stores a voltage corresponding to the data signal supplied to the first node N1 via the first transistor M1 in the period where the scan signal is supplied to the scan line Sn. When the first transistor M1 turns off, the capacitor C maintains the state in which the second transistor M2 is turned on in one frame.
The gate electrode of the second transistor M2 is connected to the first node N1, which is commonly connected to the drain electrode of the first transistor M1 and the capacitor C. The source electrode of the second transistor M2 is connected to the first power source ELVDD, and the drain electrode of the second transistor M2 is connected to the anode of the OLED. The second transistor M2 controls the amount of current supplied from the first power source ELVDD to the OLED in accordance with the data signal. Therefore, the OLED emits light by the current supplied from the first power source ELVDD via the second transistor M2.
To drive the pixel 11, the first transistor M1 is turned on in the period where a low level scan signal is supplied to the scan line Sn. Therefore, the data signal from the data line Dm is supplied to the gate electrode of the second transistor M2 via the first transistor M1 and the first node N1. At this time, the capacitor C stores the difference in voltage between the gate electrode of the second transistor M2 and the first power source ELVDD.
The second transistor M2 is turned on in accordance with the voltage of the first node N1 to supply current corresponding to the data signal to the OLED. Therefore, the OLED emits light according to the current supplied by the second transistor M2 to display images.
Then, in the period where a high level scan signal is supplied to the scan line Sn, the second transistor M2 is maintained to be turned on by the voltage corresponding to the data signal stored in the capacitor C so that the OLED emits light in one frame to display images.
The common light emitting display may additionally include a compensating circuit that compensates for non-uniform threshold voltages Vth of the second transistors M2 caused during fabrication. The light emitting display having the compensating circuit may utilize an offset compensating method or a current programming method, which have limitations on displaying uniform images.